Catalysts for the polymerization of olefins are well known in the art and have been known at least since U.S. Pat. No. 3,113,115 which described the basic Ziegler catalyst. Since that time, scores of patents have issued relating to new or improved Ziegler-type catalysts. Examples of these patents include U.S. Pat. Nos. 3,793,295; 3,850,342; 4,107,413; 4,294,721; 4,187,196, 4,226,741; 4,390,454; 4,495,338; 4,439,540; and 4,547,476. The disclosures of which, together with the disclosure of U.S. Pat. No. 3,113,115, are hereby incorporated by reference.
These patents disclose Ziegler-type catalysts that are basically a complex derived from a halide of a transition metal, for example, titanium, chromium and vanadium and a metal hydride or a metal alkyl, typically an organoaluminum compound which serves as a cocatalyst. Usually, the catalyst includes a titanium halide compound which is supported on a magnesium halide complexed with an alkyl aluminum. Also employed with Zeigler-type catalysts are electron donors or Lewis bases which aid in controlling the sterospecificity of the polymer.
It has also become known in the art that the efficiency of these types of catalysts may be significantly increased by pre-coating them with a polymeric substance in a pre-polymerization step prior to introducing the catalyst into a polymerization reaction zone. Methods for pre-coating or pre-polymerizing olefin catalysts are disclosed in U.S. Pat. Nos. 3,893,989; 4,200,717; 4,287,328; 4,316,966 and British Patent No. 1,580,635, the disclosures of which are hereby incorporated by reference. Other methods are also known in the art. All of the methods for pre-polymerizing are essentially a polymerization reaction on a small scale. The polymer product in the pre-polymerization attaches to or coats the catalyst as the polymer is formed. By pre-polymerizing, it is believed that the number of activated sites on the catalyst is increased and the morphology of the catalyst is protected as the catalyst enters a polymerization reaction zone. Instead of disintegrating upon entering the reaction zone, the catalyst is held together by the pre-polymerizing, thereby increasing the effective volume within the reactor and reducing negative downstream effects. Pre-polymerizing also enables better isotactic control of the polymer product and provides a product with better bulk density.
It has been surprisingly found that the efficiency of these pre-polymerized catalysts can be increased even further by contacting the pre-polymerized catalyst with a cocatalyst or a mixture of cocatalyst and electron donor prior to the catalyst entering the reaction zone as taught by the present invention. Increases in efficiency of almost 10,000 (gms product/gm catalyst for 2 hours) have been achieved, and further increases are possible.